The present invention relates to perforated sheet material used in vacuum hold down systems and to vacuum hold down systems which utilize said perforated sheets. Such vacuum hold down systems are used in the cutting of sheet material such as cloth and leather in connection with production of clothing, upholstery and the like.
In operation, a vacuum hold down system provides for a reduced pressure on the bottom side of a perforated sheet. When a sheet of workpiece material is laid on the topside of the perforated sheet, the vacuum draws the sheet workpiece material down against the table and acts to resist lateral motion of the workpiece across the table, even under the influence of forces resulting from cutting. In the prior art of which I am aware, hold down surfaces for use with fabrics and impermeable sheet material such as leather have included straight rows of relatively uniformly spaced holes wherein the holes have an average diameter of about 0.013 inch, and the space between the holes is about 0.048 inch between the hole centers and the wall thickness between adjacent holes is about 0.035 inch. The spacing between the lines of holes is about 0.5 inches.
Prior art vacuum systems with uniformly distributed holes waste some of the available vacuum because a substantial fraction of the holes are not covered by the workpiece.
In the prior art vacuum surface sheets, the perforated sheets have tended to fail under the influence of the downward pressure of the cutting tool. Failures have generally occurred along the straight lines of spaced holes.